Understanding the 0.2 percent offset yield strength method

Why use 0.2% offset yield strength?

Before we begin learning about the 0.2% offset yield strength method let’s take a moment different types of deformation in materials.

  • Elastic deformation

This refers to the temporary deformation that occurs in a material when stress is applied to it. by temporary deformation, we mean that the deformed material can easily revert back to its original shape. To recover back to the original shape only the applied load needs to be removed. Elastic deformation is mainly studied in materials that undergo minute and small deformations

  • Plastic deformation

Whenever there is a load on the material the natural effect on it is that its shape changes because it deforms. Beyond a certain point, the material can even fracture. When the deformation is non-recoverable meaning the material cannot be reverted back to its original shape automatically if the stress is removed then that kind of deformation is called plastic deformation. It leads to strain hardening, necking and fracture. Hooke’s law governs the elastic region and plastic deformation. Hooke’s law is a relation between the applied stress and elastic deformation within the bounds of the elastic limit.

This law also uses a constant called Young’s modulus.

Elastic limit is observed marked at the point when there are signs of forced formation. However, do know that while calculating elastic limit only signs of first plastic deformation are accounted for. What this means is that at this point the behaviour of the material ranges from being elastic to plastic which in turn means non-recoverable deformation appears. As opposed to plastic deformation as long as the material stays within the elastic limit it would and it can return to the original size or shape. However, on the plastic limit, a permanent deformation appears.

What is yield strength?

A very important concept while studying deformation is yield strength. Yield strength basically refers to the stress amount that could lead to a potential permanent deformation. Hence by the time the stress reaches the yield strength levels, small amounts of permanent deformation already appear in the material. This means that the yield point lies just beyond the elastic limit. However, in the case of ductile materials, the yield point is not very well defined because the stress-to-strain curve is very gradual and the change from elastic to plastic cannot be reflected in discrete points but more in terms of the continuous graph. What is 0.2% offset yield strength? It is the stress value that equates to 0.2% plastic strain. It is also referred to as proof stress.

What is the importance of the 0.2% offset yield strength method?

This concept is mainly employed to find out the exact point when the nature of the material changes from elastic to plastic. The elastic and plastic regions are differentiated using the 0.2% offset yield strength.

Material suppliers use the 0.2% offset value and material certificates. This is important so the buyers may know about the load-carrying capacity of the material while you are using it in construction or for any other purposes. The offset value for fragile materials is usually between 0.05% and 0.1%. The value is low in fragile materials because in these materials the plastic deformation is very small.

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